Photoelectric detection structure

ABSTRACT

The photoelectric device comprises a photosensitive layer on a substrate which is transparent to incident radiation. An intermediate layer for optically adapting the photosensitive layer to the substrate is provided therebetween. The respective thicknesses of the intermediate layer and the photosensitive layer are proportioned so that photon absorption takes place in the photosensitive layer near the output of the layer within a distance on the order of magnitude of the escaping depth of the electrons. Photon absorption takes place in such manner that the efficiency of the photoemission of the structure is optimum taking into account the nature of the materials of the layers.

BACKGROUND OF THE INVENTION

The invention relates to a photoelectric detection device for radiationhaving wavelengths in a given range of the spectrum. The devicecomprises, in an evacuated envelope, a photosensitive layer which isprovided on a substrate which is transparent to incident radiation. Thedevice further comprises an intermediate layer for optical adaptation.The intermediate layer is also transparent to the incident radiation,and it is provided between the photosensitive layer and the substrate.The refractive index of the material of the intermediate layer isbetween that of the substrate and that of the material of thephotosensitive layer. Such devices may be, for example, photoelectriccells, image intensifier tubes, display tubes integrated in televisionpick-up systems, or photomultipliers.

When a photoelectric detection device comprises a photosensitive layerprovided directly on a substrate, this results in general in a pooroptical adaptation of the photosensitive layer to the substrate. As aresult, a large part of the light incident on the substrate is noteffectively used for the conversion of photons into electrons. Thephotoelectric detection efficiency of the device is thereby considerablyreduced. It is known to improve the efficiency of such a device byattenuating the reflections formed at the interface between thesubstrate and the photosensitive layer. The reflections are attenuatedby means of one or more intermediate layers which are transparent to theincident radiation and which are disposed between the substrate and thephotosensitive layer.

Such a device having only one single intermediate layer is known, forexample, from U.S. Pat. No. 3,254,253 (Davis, et al). The intermediatelayer in this case has been chosen for its weak absorption. Moreover,the optical constants and the thickness of the intermediate layer arechosen such that, taking into account the optical constants of thesubstrate and of the photosensitive layer, the reflected light rays atthe interface between the substrate and the intermediate layer and thereflected rays at the interface between the intermediate layer and thephotosensitive layer, respectively, have exactly the same amplitude andopposite phases so that they neutralize each other by interference.

Such a device attenuates the losses as a result of reflections to aconsiderable extent but does not necessarily result in a device havingan optimum efficiency.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a photoelectric detectiondevice which comprises a photosensitive layer supported by a substratewhich is transparent to incident light. The device also has atransparent intermediate layer between the substrate and thephotosensitive layer. The efficiency of the device is optimized takinginto account the nature of the materials of the substrate and of thephotosensitive and intermediate layers, respectively.

According to the invention, in a device of the kind described above thethickness e of the photosensitive layer and the thickness e₁ of theintermediate layer are proportioned so that the absorption of thephotons in the relevant range of the spectrum takes place in a portionof the photosensitive layer near the interface of the layer with thevacuum in the device. This portion of the photosensitive layer, startingfrom the interface, has a thickness on the order of magnitude of theescaping depth L of the produced photoelectrons.

The invention can be understood with reference to the theoreticalformulae of the efficiency of photoemission of a photoelectric detectiondevice, with or without an intermediate layer between the substrate andthe photosensitive layer. The absorption of the light is assumed to takeplace in the photosensitive layer.

If no intermediate layer is present, the efficiency depends on thethickness of the photosensitive layer and on the optical constantsthereof n k (n is the refractive index and k and the extinction index ofthe material). The formula for the efficiency reads as follows. ##EQU1##wherein the symbols used have the following meanings;

x=the distance between (i) the interface of the photosensitive layer andthe vacuum of the tube (x=0 at this interface), and (ii) the place ofabsorption of the photons in the layer measured at right angles to theinterface;

W=the energy of the photoelectrons;

A (n,k,x)=the absorption function of the radiation of wavelength λ inthe photosensitive layer at the distance x from the interface betweenthe photosensitive layer and the vacuum;

P (W,O)=the escaping probability of the photoelectrons at the interface(equal to unity in the following applications);

L=the escaping depth of the photoelectrons from the photosensitivelayer;

f (x,L)=the formula which represents the transport of the electrons inthe layer;

e=the thickness of the photosensitive layer.

With an intermediate layer having a thickness of e₁ and with opticalconstants n₁ and k₁ (n₁ is the refractive index and k₁ is the extinctionindex) located between the substrate and the photosensitive layer, theabsorption function Aλ of the photons in the photosensitive layerdepends not only on n, k and x but also on e₁, n₁ and k₁. Thus theformula for the efficiency Pλ of the photoemission of the modifiedstructure reads as follows. ##EQU2##

Application of the formulae (1) and (2) and the formula for thetransport of the electrons f (x,L)=e (-X/L) (where e denotes theNeperian number) for a few theoretical cases, and for a few experimentalexamples serves to illustrate the invention. The materials used for thesubstrate have a refractive index on the order of magnitude of 1.5 to 2and those for the intermediate layer with transparent material (k₁ #0)have a refractive index larger than that of the substrate and smallerthan that of the photosensitive layer.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partly schematic, partly sectional view of the photoelectricdetection device according to the invention.

FIG. 2 is a graph showing a number of curves indicating the efficiencyof the photoemission of the device as a function of the thickness e ofthe photosensitive layer for various values, e₁, of the intermediatelayer. All curves are at a wavelength, λ, =4360 Å. The photoemissivematerial is (Sb Na₂ K, Cs) and the intermediate layer consists of TiO₂.

FIG. 3 is a graph showing a number of similar curves indicating theefficiency of the photoemission of the device at the wavelength, λ,equal to 5460 Å.

FIG. 4 is a graph showing a number of similar curves indicating theefficiency of the photoemission of the device at the wavelength, λ,equal to 8000 Å.

FIG. 5 is a graph showing a number of curves indicating the energysensitivity as a function of the thickness e of the photosensitive layerof a photoemission device with or without an intermediate layer of TiO₂having a thickness of e₁ =500 Å.

FIG. 6 is a graph showing the spectral energy sensitivity of aphotoemission device according to the invention as a function of thewavelength of the light, where the device has an intermediate layerhaving a thickness of e₁ =500 Å consisting of TiO₂, and the device hasphotosensitive layer having a thickness of e =900 Å consisting of (SbNa₂ K, Cs). FIG. 6 also shows the sensitivity of the same photosensitivelayer having a thickness of 1300 Å provided directly on a glasssubstrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a sectional view of an embodiment of a device in which thesubstrate consists of a disc 11 which is transparent to radiation. Aphotosensitive layer 12 having a thickness e, is provided on anintermediate layer 13 on the substrate 11. The intermediate layer 13 isalso transparent to radiation, and it has a thickness e₁ for the opticaladaptation between the substrate 11 and photosensitive layer 12. Thisstacked construction forms the input of a photoelectric tube in whichthe light to be detected is incident on the left-hand side of the stackin the direction of the arrow 14. The vacuum of the tube 15 is on theright-hand side of the photosensitive layer 12.

According to a first embodiment of the invention the efficiency of thephotoemission of the photosensitive layer is improved. An example ofthis embodiment includes a photosensitive layer of the type S20,trialkaline having the chemical formula (Sb Na₂ K, Cs). Thisphotosensitive layer is provided directly on a glass substrate having arefractive index on the order of magnitude of 1.5 in the blue, green andred regions of the visible spectrum centered on the wavelengths ofλ=4360 Å, λ=5460 Å, and λ=8000 Å, respectively. The efficiency ρλ of thephotoemission of such a layer is maximum in each of the wavelengthregions for a given value of the thickness e of the layer. The order ofmagnitude of this value is indicated on line 2 of Table I (below)dependent on the spectral region. The corresponding efficiency of thephotoemission is indicated on line 3 of Table I, expressed in the numberof electrons per incident photon×100%.

According to this first embodiment, the intermediate layer providedbetween, the photosensitive layer and the substrate is a layerconsisting of, for example, TiO₂ having a refractive index of 2.6. FIGS.2,3, and 4 show the variations of the efficiency as a function of thethickness e of the photosensitive layer, for the colours blue, green andred, respectively, centered on the wavelengths λ=4360 Å λ=5460 Å, andλ=8000 Å, respectively. In these figures, each curve represents a valueof e₁ of the intermediate layer. The efficiency ρ'λ of the photoemissionof the structure is optimum in each of the spectral ranges when thevalues of e and e₁ optimum correspond to the values on lines 4 and 5 ofTable I. In each case the optimum efficiency itself in line 6. On line 7is indicated the ratio ρ'λ/ρλ is shown, equal to 1.3, 1.25, and 1.1 inthe blue, green and red spectral regions, respectively. The mostimportant photoelectric gain is thus obtained in the blue light with athickness of the photocathode comparable to photosensitive layers of thetype S 20 of the same composition directly provided on the substrate.

The photoelectric detection structure according to the invention is notrestricted to that corresponding to the thicknesses e and e₁ having thevalues indicated in Table I.

                  TABLE I                                                         ______________________________________                                        BLUE             GREEN      RED                                               λ = 4360 Å                                                                          λ = 5460 Å                                                                    λ = 8000 Å                             ______________________________________                                        e (Å)                                                                             100          800        1300                                          P.sub.λ  %                                                                     40           27         13                                            e.sub.1 (Å)                                                                       1300         500        700                                           eÅ  200          400        750                                           ρ.sub.λ '%                                                                 52.5         33.5       14.5                                           ##STR1##                                                                             1.3          1.25       1.1                                           ______________________________________                                    

Moreover, as may be seen from each of the FIGS. 2, 3 and 4, other valuesof e and e₁ exist for which the efficiency of the photoemission is veryoptimum. Each pair of values corresponds to a modified embodiment of theinvention. These values are recorded in Table II, below, for each of thespectral ranges.

                                      TABLE II                                    __________________________________________________________________________          BLUE     GREEN      RED                                                 λ                                                                            λ = 4360 Å                                                                  λ = 5460 Å                                                                    λ = 8000 Å                               __________________________________________________________________________    e.sub.1 (Å)                                                                     1300 1100 500 300                                                                      1500 1300 700 500 300                                                                    1100  900  700  500  300                            e (Å)                                                                            200  350 200 300                                                                       450  600 250 400 600                                                                     400  500  750  950 1100                            e + e.sub.1 (Å)                                                                 1500 1450 700 600                                                                      1950 1900 950 900 900                                                                    1500 1400 1450 1450 1400                            __________________________________________________________________________

In the blue and green spectral regions, the values of e and e₁ aregrouped in two combinations, each corresponding to a sum of thethicknesses e and e₁ which are very constant. That is to say, in theblue range e+e₁ =1450 Å and e+e₁ =700 Å, and in the green range e+e₁=1900 Å, e=e₁ =900 Å. In the red range the value pairs form acombination for which it holds that e+e₁ =1450 Å. Taking into accountthe accuracy of the measurements, the invention also includes structuresfor which the sum e+e₁ is approximately equal to the above-mentionedvalues within a tolerance range of ±15%. For spectral ranges havingdifferent wavelengths, the invention provides devices which are definedin an analogous manner and the sums e+e₁ of which are characteristic ofthe spectral ranges described.

A second embodiment according to the invention consists of photoelectricdetection devices for use in the visible and in the near infraredspectra, while maintaining the sensitivity in these spectra as uniformas possible. The device chosen has, for example, a photosensitive layerof (Sb Na₂ K, Cs) and an intermediate layer of TiO₂. The thicknesses eand e₁ are on the order of magnitude of e₁ =500 Å and e=900 Å.

FIG. 5 shows three pairs of curves denote by B, G, and R. These curvesdenote the energy sensitivity in milliamperes per Watt of thephotoelectric detection structures in the blue, green and red regions ofthe spectrum, respectively, dependent on the thickness, e, of thephotosensitive layer. The broken line curves relate to a device having aphotosensitive layer provided directly on the substrate, the solid linecurves relate to a device having a photosensitive layer on anintermediate layer having a thickness e₁ =500 Å. The probability P (W,O) of the escape of the electrons from the photosensitive layer beingassumed to be equal to 0.5 in both cases.

These curves of FIG. 5 make it possible to compare the sensitivity ofthe photoelectric device according to the invention with that of aphotosensitive layer directly on the glass and the thickness of whichmust be 1300 Å. This photoelectric amplification according to theinvention with respect to the layer directly on the substrate is on theorder of magnitude of 1.1 for the red, 1.5 for the green, and 2.5 forthe blue.

The consequences of the photoelectric amplification according to theinvention are indicated by means of the curves 61 and 62 in FIG. 6.These curves show the variations of the energy sensitivity expressed inmA per Watt as a function of the wavelength of the incident light forthe photosensitive layer directly on the substrate and for the structureaccording to the invention, respectively. With respect to the formerlayer, the sensitivity for the blue and the green becomes larger andthus presents a certain uniformity.

Of course the invention also includes all devices in which aphotosensitive layer and a transparent intermediate layer (K_(1#) 0) isprovided on a substrate, the refractive index of the intermediate layerbeing between that of the substrate and that of the photosensitivematerial.

In this manner the photosensitive layer according to the modifiedembodiments is bialkaline according to the chemical formula Sb Ax By(where A and B are alkali metals and x, y are coefficients) when it isconcerned with increasing the sensitivity in the blue and the greenregions, or according to the chemical formula Sb, Ax when it isconcerned with increasing the sensitivity only in the blue region, oraccording to the chemical formula Ag O Sc when it is concerned withincreasing the sensitivity in the whole visible spectrum and in the rearinfrared spectrum.

In addition the material TiO₂ of the intermediate layer may be, forexample, replaced by Ta₂ O₅ or also In₂ O₃ or SnO₂ (except in thepresence of sodium) or SiO, MnO, Al₂ O₃, Si₃ N₄, MgO or also lanthanumglass provided in a thin layer. When the materials of the photosensitivelayer and of the intermediate layer are as stated above, the thicknessese and e₁ of the photosensitive layer and the intermediate layer havesubstantially the same values as those indicated in Tables I and II, inwhich deviations of 15% are permitted without considerably deviatingfrom the optimum value of the efficiency of the photoemission of thedevice.

Among the other advantages indicated for the device according to theinvention are the small thicknesses of the photosensitive layer ascompared with that of prior art devices. In addition, that certainintermediate layers, for example SnO₂ and In₂ O₃, stabilize the electricpotential on the surface of the photosensitive layer when the devicesare used as photocathodes due to a very low electric resistance.

What is claimed is:
 1. A photoelectric detection device for detectingradiation in a given spectral region, said device comprising:an envelopeenclosing an evacuated space; a substrate on the envelope, saidsubstrate being transparent to radiation in the spectral region; anintermediate layer provided inside the envelope on the substrate, saidintermediate layer being transparent to radiation in the spectralregion; and a photosensitive layer provided inside the envelope on aside of the intermediate layer opposite to the substrate, saidphotosensitive layer having a first side adjacent to the intermediatelayer and a second side opposite to the first side, said photosensitivelayer having an escaping depth of photoelectrons; characterized in that:the substrate, the intermediate layer, and the photosensitive layer eachhave a refractive index, the refractive index of the intermediate layerbeing between the refractive index of the substrate and the refractiveindex of the photosensitive layer; and the intermediate layer and thephotosensitive layer each have thicknesses chosen so that absorption ofradiation in the spectral region occurs substantially in a portion ofthe photosensitive layer adjacent to the second side of thephotosensitive layer and said portion having a thickness on the order ofmagnitude of the escaping depth of photoelectrons in the photosensitivelayer.
 2. A photoelectric detection device as claimed in claim 1,characterized in that:the substrate comprises glass having a refractiveindex on the order of magnitude of 1.5; the photosensitive layerconsists essentially of a material from the group consisting of SbA_(x)B_(y) Cs, SbA_(x) B_(y), SbA_(x), and AgOCs, where A and B are alkalimetals and x and y are between 0 and 3; and the intermediate layer has arefractive index between 1.9 and 2.6 and is chemically compatible withthe photosensitive layer.
 3. A photoelectric detection device as claimedin claim 2, characterized in that:the thickness of the intermediatelayer is 1300 angstroms ±195 angstroms; and the thickness of thephotosensitive layer is 200 angstroms ±30 angstroms.
 4. A photoelectricdetection device as claimed in claim 2, characterized in that:thethickness of the intermediate layer is 1100 angstroms ±165 angstroms;and the thickness of the photosensitive layer is 350 angstroms ±52.5angstroms.
 5. A photoelectric detection device as claimed in claim 2,characterized in that:the thickness of the intermediate layer is 500angstroms ±75 angstroms; and the thickness of the photosensitive layeris 200 angstroms ±30 angstroms.
 6. A photoelectric detection device asclaimed in claim 2, characterized in that:the thickness of theintermediate layer is 300 angstroms ±45 angstroms; and the thickness ofthe photosensitive layer is 300 angstroms ±45 angstroms.
 7. Aphotoelectric detection device as claimed in claim 2, characterized inthat:the thickness of the intermediate layer is 1500 angstroms ±225angstroms; and the thickness of the photosensitive layer is 450angstroms ±67.5 angstroms.
 8. A photoelectric detection device asclaimed in claim 2, characterized in that:the thickness of theintermediate layer is 1300 angstroms ±195 angstroms; and the thicknessof the photosensitive layer is 600 angstroms ±90 angstroms.
 9. Aphotoelectric detection device as claimed in claim 2, characterized inthat:the thickness of the intermediate layer is 700 angstroms ±105angstroms; and the thickness of the photosensitive layer is 250angstroms ±37.5 angstroms.
 10. A photoelectric detection device asclaimed in claim 2, characterized in that:the thickness of theintermediate layer is 500 angstroms ±75 angstroms; and the thickness ofthe photosensitive layer is 400 angstroms ±60 angstroms.
 11. Aphotoelectric detection device as claimed in claim 2, characterized inthat:the thickness of the intermediate layer is 300 angstroms ±45angstroms; and the thickness of the photosensitive layer is 600angstroms ±90 angstroms.
 12. A photoelectric detection device as claimedin claim 2, characterized in that:the thickness of the intermediatelayer is 1100 angstroms ±165 angstroms; and the thickness of thephotosensitive layer is 400 angstroms ±60 angstroms.
 13. A photoelectricdetection device as claimed in claim 2, characterized in that:thethickness of the intermediate layer is 900 angstroms ±135 angstroms; andthe thickness of the photosensitive layer is 500 angstroms ±75angstroms.
 14. A photoelectric detection device as claimed in claim 2,characterized in that:the thickness of the intermediate layer is 700angstroms ±105 angstroms; and the thickness of the photosensitive layeris 750 angstroms ±112.5 angstroms.
 15. A photoelectric detection deviceas claimed in claim 2, characterized in that:the thickness of theintermediate layer is 500 angstroms ±75 angstroms; and the thickness ofthe photosensitive layer is 950 angstroms ±142.5 angstroms.
 16. Aphotoelectric detection device as claimed in claim 2, characterized inthat:the thickness of the intermediate layer is 300 angstroms ±45angstroms; and the thickness of the photosensitive layer is 1100angstroms ±165 angstroms.
 17. A photoelectric detection device asclaimed in claim 2, characterized in that:the photosensitive layerconsists essentially of a material from the group consisting of SbK₂ Cs,SbK₂ Rb, SbRb₂ Cs, SbCs₃, and AgOCs; and the intermediate layer consistsessentially of a material from the group consisting of TiO₂, Ta₂ O₅, In₂O₃, SnO₂, SiO, MnO, Al₂ O₃, Si₃ N₄, MgO, and lanthanum glass.
 18. Aphotoelectric detection device as claimed in claim 17, characterized inthat:the thickness of the intermediate layer is 500 angstroms ±75angstroms; and the thickness of the photosensitive layer is 900angstroms ±135 angstroms.
 19. A photoelectric detection device asclaimed in claim 2, characterized in that:the photosensitive layerconsists essentially of a material from the group consisting of SbK₂ Cs,SbNa₂ KCs, SbK₂ Rb, SbRb₂ Cs, SbCs₃, and AgOCs; and the intermediatelayer consists essentially of a material from the group consisting ofTiO₂, Ta₂ O₅, SiO, MnO, Al₂ O₃, MgO, Si₃ N₄, and lanthanum glass.
 20. Aphotoelectric detection device as claimed in claim 19, characterized inthat:the thickness of the intermediate layer is 500 angstroms ±75angstroms; and the thickness of the photosensitive layer is 900angstroms ±135 angstroms.